The use of an ion exchange membrane in an electrolytic cell to produce an alkali metal hydroxide by electrolyzing an aqueous solution of an alkali metal chloride is well known. Much of the technology in this area has focused on improvements to the materials, composition and configuration of the ion exchange membranes, so as to reduce cell voltage and increase current efficiency but maintain the strength and durability of the membrane.
It is possible to apply a porous non-electrode layer on at least one side of a cation exchange membrane for electrolysis of an alkali metal chloride solution. The coating reduces the cell voltage. The coating may be made of metal oxides, metal carbides and the like.
The prior art also discloses the use of a membrane with at least one roughened surface which contacts or is located within 20 mm of the corresponding electrode. The advantage taught is the reduction of cell voltage. While the prior art discloses methods of roughening the surface, there is no mention of improvement to current efficiency of the cell. In addition, the prior art does not mention the time or extent of sanding or roughening required to improve the membrane. The prior art also does not contemplate the importance of baring any, let alone a certain proportion, of the crossover points of the reinforcing fabric as a criterion of the desirable degree of roughening.
It is also possible to introduce channels on the anolyte surface of the membrane. The channels are achieved by using a mixture of reinforcing and sacrificial fibers, and later removing the sacrificial fibers. The channel-containing membrane does not, however, achieve the increase in current efficiency of the present invention. Further, the method of making this channel-containing membrane is defective because it is not possible to make the desired "degree of openness" reproducibly.
Both cell voltage and current efficiency are important in the electrolysis of metal chloride solutions in order to reduce the consumption of electric power, a major factor in the economics of chloralkali production. Cell voltage should be minimized and current efficiency should be maximized. Because of the importance of power consumption and the large scale on which this electrolysis is conducted, one of the problems to be solved is to achieve even a small increase in current efficiency without any increase in cell voltage.
In narrow gap and zero gap cells, cell shutdowns occasionally occur, sometimes resulting in undesirable contamination of the membrane with Nickel or other metals picked up from the cathode. This problem is greatly reduced by the present invention.
Variations in voltage of the electrolytic cell represent another problem. For example, heat balance to a chloralkali cell can be upset by voltage cycles of as little as 30-50 mV. If constant temperature is not maintained, the membrane will not operate at equilibrium condition, and current efficiency can be undesirably affected. Furthermore, small changes in voltage can contribute to unsteady current density within the membrane. This, too, can lead to undesirable decline in current efficiency. In systems operating with multiple electrolytic cells, current will be shunted to other cells operating at lower voltage. This adversely changes the current balance, heat balance, and ultimately performance of the cell. Finally, if the membrane is not operated in equilibrium, other operating variables become more difficult to control. For instance, water transport can be effected, and as a result, undesired changes in caustic concentration and current efficiency can occur. The problem of voltage variability is greatly reduced by the present invention. These problems and other problems apparent to one skilled in the art are solved by the present invention, without forfeiting the advantages of the prior art.